FIELD OF THE INVENTION
[0001] This invention relates to an apparatus for generating an address to access a memory,
and more particularly to a micro-computer utilizing a segment mode for access of a
memory.
BACKGROUND OF THE INVENTION
[0002] A conventional apparatus utilizing a segment mode for generating an address to access
a memory comprises a physical address calculation unit for generating a physical address
of 20 bits from a segment address of 16 bits and an offset address of 16 bits. In
this generation of the physical address, lower four bits are added to the segment
address of 16 bits, and upper four bits are added to the offset address of 16 bits,
so that the segment and offset addresses are provided with a shift of 4 bits therebetween
and are added to generate the physical address of 20 bits. Thus, a memory space of
1 M bytes can be accessed by the physical address of 20 bits.
[0003] In this segment mode, a memory space is divided into a plurality of logic segments
having arbitrary memory sizes. In the logic segments, a start address of a corresponding
logic segment is a segment value which is a segment address, and an offset address
thereof is an offset value from the start address. Thus, a physical address is generated
by the addition of the segment and offset addresses.
[0004] However, the conventional apparatus utilizing the segment mode for generating an
address to access a memory has a disadvantage in that a size of a memory space which
is accessed by a physical address is fixed, because the number of bits which are added
to a segment address and an offset address, that is, a shift amount provided between
the two addresses is fixed. As a result, it is impossible that the size of the memory
space is increased as much as desired by customers.
SUMMARY OF THE INVENTION
[0005] Accordingly, it is an object of the invention to provide an apparatus for generating
an address signal to access a memory, in which a space of a memory space which is
accessed by a physical address can be varied.
[0006] According to this invention, an apparatus for generating an address to access a memory,
comprises:
means for generating an offset address by receiving a first control signal generated
by decoding a command code;
means for generating a segment address by receiving a second control signal generated
by a predetermined command included in the command code; means for providing a
shift of predetermined bits between the offset and segment addresses, the shift being
determined to be varied dependent on a third control signal generated by decoding
the command code; and
means for adding the offset and segment addresses having the shift therebetween
to generate a physical address.
BRIEF DESCRIPTION OF THE DRAWINGS
[0007] This invention will be explained in more detail in conjunction with appended drawings,
wherein:
Fig. 1 is a block diagram showing a conventional apparatus for generating an address
to access a memory;
Fig. 2 is a block diagram showing a physical address calculation processor in the
conventional apparatus in Fig. 1;
Figs. 3 and 4 are explanatory diagrams showing the addition of offset and segment
addresses in the physical address calculation processor in Fig. 2;
Fig. 5 is a block diagram showing an apparatus for generating an address to access
a memory in a first preferred embodiment according to the invention;
Fig. 6 is a block diagram showing a segment address selector and a physical address
calculation processor in the first preferred embodiment;
Figs. 7 and 8 are explanatory diagrams showing the addition of offset and segment
addresses in the physical address calculation processor in Fig. 6;
Fig. 9 is a block diagram showing an apparatus for generating an address to access
a memory in a second preferred embodiment according to the invention;
Fig. 10 is a block diagram showing a segment address selector and a physical address
calculation processor in the second preferred embodiment;
Figs. 11 to 13 are explanatory diagrams showing the addition of offset and segment
addresses in the address calculation processor in Fig. 10.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0008] Before explaining an apparatus for generating an address to access a memory in the
first preferred embodiment according to the invention, the aforementioned conventional
apparatus utilizing the segment mode will be explained in Fig. 1.
[0009] This conventional apparatus comprises a bus control unit 110 for controlling buses
by receiving a control signal 140A, a micro-sequencer 140 for generating control signals
including the control signal 140A to control internal units in a micro-program control
method by reading command codes from the bus control unit 110, to which the command
codes are written from a memory (not shown) by the bus control of the bus control
unit 110, an arithmetic logic unit 120 for carrying out various arithmetics including
the calculation of an offset address by receiving a control signal 140B from the micro-sequencer
140, a physical address calculation unit 150 for generating a physical address in
the addition of the offset address and a segment address (to be explained later) by
receiving a calculation conduct signal 140C from the micro-sequencer 140, and a register
file 130 having a set of registers, one of which stores the segment address and is
selected by a register designation signal 140D.
[0010] The bus control unit 110, the arithmetic logic unit 120, the register file 130, and
the physical address calculation unit 150 are connected to each other by an internal
bus 160, and the bus control unit 110 is connected to the memory, etc. by an external
bus 170.
[0011] The micro-sequencer 140 comprises a micro-program controller 141 for generating control
signals including the control signals 140A, 140B and 140C, and a register reference
signal 141A in the micro-program method, a prefix controller 142 for carrying out
prefix operation, when a command code supplied from the bus control unit 110 is a
prefix command, and a register selector 143 for selecting one register in the register
file 130 by receiving a segment override prefix signal 142A generated in case that
the prefix command supplied to the prefix controller 142 is a segment override prefix
command, the register reference signal 141A, and a command code supplied from the
bus control unit 110.
[0012] The physical address calculation unit 150 comprises an offset address latch 151 for
latching an offset address supplied from the arithmetic logic unit 120, a segment
address latch 152 for latching a segment address supplied from a register of the register
file 130 selected by the register designation signal 140D supplied from the register
selector 143, and a physical address calculation processor 153 for adding contents
of the offset and segment address latchs 151 and 152 to generate a physical address.
[0013] In the micro-sequencer 140, a prefix command which is included in command codes supplied
from the bus control unit 110 to the prefix controller 142 is pre-positioned to a
calculation command or a transfer command to change a basic operation of the command,
and is defined as a modified command. In a transfer command of data between a memory
and a register, for instance, a segment override prefix command is set to replace
a segment register in the register file 130 by another segment register therein.
[0014] In operation, it is assumed that a memory space is of 1 M bytes, and a segment address
and an offset address are of 16 bits, respectively, in the segment mode, and a physical
address is of 20 bits.
[0015] A memory operation command including a segment override prefix command is supplied
from the memory to the micro-sequencer 140 by the bus control of the bus control unit
110. In the micro-sequencer 140, a segment override prefix signal is supplied from
the prefix controller 142 to the register selector 143 by the segment override prefix
command.
[0016] On the other hand, a register reference signal 141A is supplied from the micro-program
controller 141 to the register selector 143, so that a register designation signal
140D is supplied from the register selector 143 to the register file 130 to select
a predetermined segment register in accordance with the segment override prefix signal
142A, the register reference signal 141A, and a command code of the segment override
prefix command. Consequently, a segment address is read from the selected segment
register of the register file 130, and is supplied therefrom via the internal bus
160 to the physical address calculation unit 150, in which the segment address is
latched in the segment address latch 152.
[0017] Simultaneously, an offset address is calculated in the arithmetic logic unit 120
in reference to a portion of command codes written into the bus control unit 110 by
receiving a control signal 140B supplied from the micro-program controller 141. The
calculated offset address is supplied from the arithmetic logic unit 120 through the
internal bus 120 to the physical address calculation unit 150, in which the offset
address is latched in the offset address latch 151.
[0018] In the physical address calculation unit 150, the offset address latched in the offset
address latch 151 and the segment address latched in the segment address latch 152
are added in the physical address calculation processor 153 to provide a physical
address by receiving a physical address calculation conduct signal 140C.
[0019] Fig. 2 shows the physical address calculation processor 153 which comprises first
and second bit shifters 153A and 153B for adding upper and lower four bits to the
offset and segment addresses, correspondingly, and an adder 153C for adding the bit-shifted
offset and segment addresses to provide the physical address. In this physical address
calculation processor 153, the upper four bits are added to the offset address of
16 bits supplied from the offset address latch 151 in the first bit shifter 153A to
provide the bit-shifted offset address of 20 bits, and the lower four bits are added
to the segment address of 16 bits supplied from the segment address latch 152 in the
second bit shifter 153A to provide the bit shifted segment address of 20 bits, so
that the bit-shifted offset and segment addresses are added in the adder 153C to provide
the physical address of 20 bits, as shown in Fig. 3, by which a memory space of 1
M bytes is accessed. Thus, the memory is accessed in accordance with the physical
address of 20bits by the bus control unit 110.
[0020] In the shift between offset and segment addresses, if eight bits are shifted as shown
in Fig. 4, a physical address of 24 bits is generated to access a memory space of
16 M bytes.
[0021] At any rate, the number of shifting bits is fixed in the conventional apparatus.
Therefore, the aforementioned disadvantage occurs therein.
[0022] Next, an apparatus for generating an address signal to access a memory in the first
preferred embodiment according to the invention will be explained in Fig. 5, wherein
like parts are indicated by like reference numerals as used in Fig. 1.
[0023] In this address signal generating apparatus, the physical address calculation unit
150 comprises a segment address selector 154 for selecting one of first and second
bit-shifted segment addresses by receiving a segment address selecting signal 143A
from the register selector 143 in the micro-sequenser 140, in addition to the offset
and segment address latchs 151 and 152 and the physical address calculation processor
153 as explained in Fig. 1. The rest of this apparatus is the same as in Fig. 1.
[0024] Fig. 6 shows the physical address calculation unit 150 which comprises the offset
address latch 151, the segment address latch 152, the physical address calculation
processor 153, and the segment address selector 154, wherein the segment address selector
154 includes a first bit shifter 154A for adding upper bits to the segment address
of 16 bits, a second bit shifter 154B for adding lower bits to the segment address
of 16 bits, and an inverter 154C for inverting the segment address selecting signal
143A supplied from the register selector 143.
[0025] In this first preferred embodiment, the register file 130 comprises a set of registers,
among which a first segment register effective for accessing a memory space of the
least significant 1 M bytes of 16 M bytes, and a second segment register effective
for accessing a memory space of 16 M bytes.
[0026] In operation, it is assumed that the first segment register is used.
[0027] In the same manner as in the conventional apparatus, the offset and segment addresses
of 16 bits are latched in the offset and segment address latchs 151 and 152, respectively.
Then, the segment address is supplied from the segment address latch 152 to the segment
address selector 154, in which upper four bits are added to the segment address in
the first bit shifter 154A to provide a first bit-shifted segment address of 20 bits,
and lower four bits are added to the segment address in the second bit shifter 154B
to provide a second bit-shifted segment address of 20 bits. Here, it is assumed that
the segment address selecting signal is "0". Therefore, the first bit-shifted segment
address is supplied from the bit shifter 154A to the physical address calculation
processor 153. In the physical address calculation processor 153, upper eight bits
are added to the offset address of 16 bits supplied from the offset address latch
151 in the first bit shifter 153A to provide a bit-shifted offset address of 24 bits,
and lower four bits are added to the first bit-shifted segment address of 20 bits
supplied from the segment address selector 154 in the second bit shifter 153B to provide
a third bit-shifted segment address of 24 bits. Then, the bit shifted offset address
and the third bit-shifted segment address having a shift of 4 bits therebetween are
added in the adder 153C to provide a physical address of 20 bits, by which a memory
space of 1 M bytes is accessed, as shown in Fig. 7.
[0028] Next, it is assumed that the second segment register is used in operation in the
first preferred embodiment. In case of using the second segment register, a segment
address selecting signal 143A of "1" is supplied to the segment address selector 154,
so that the second bit-shifted segment address of 20 bit is supplied from the second
bit shifter 154B of the segment address selector 154 to the physical address calculation
processor 153, in which the upper eight bits are added in the first bit shifter 153A
to the offset address supplied from the offset address latch 151, and lower four bits
are added in the second bit shifter 153B to the second bit shifted segment address.
Then, the offset and segment addresses having a shift of 8 bits therebetween supplied
from the first and second bit shifters 153A and 153B added in the adder 153C to provide
a physical address of 24 bits, by which a memory space of 16 M bytes is accessed,
as shown in Fig. 8.
[0029] Fig. 9 shows an apparatus for generating an address to access a memory in the second
preferred embodiment according to the invention, wherein like parts are indicated
by like reference numerals as used in Fig. 5.
[0030] In this apparatus, the register file 130 comprises a set of registers including a
plurality of segment registers such as a first segment register effective for accessing
a memory space of 1 M bytes, a second segment register effective for accessing a memory
space of 16 M bytes, a third segment register effective for accessing a memory space
of 256 M bytes, etc., and the segment address selector 154 of the physical address
calculation unit 150 is controlled by a segment address selecting signal 143A' of
2 bits supplied from the register selector 143 of the micro-sequencer 140.
[0031] Fig. 10 shows the segment address selector 154 which comprises first to third bit
shifters 154D, 154E and 154F, and a decoder 154G for decoding the segment address
selecting signal 143A'.
[0032] In operation, it is assumed that the first segment register is used, and the segment
address selecting signal 143A' is "00".
[0033] In the offset and segment address latchs 151 and 152, offset and segment addresses
each having 16 bits are set to be supplied to the bit shifter 153A of the physical
address calculation processor 153 and to the first to third bit shifters 154D, 154E
and 154F of the segment address selector 154, correspondingly. In the segment address
selector 154, the first bit shifter 154D is selected as a result of decoding the selecting
signal 143A' of "00", so that the segment address of 24 bits having added upper 8
bits is supplied to the second bit shifter 153B of the physical address calculation
processor 153, in which lower 4 bits are added thereto. On the other hand, upper 12
bits are added to the offset address of 16 bits in the first bit shifter 153A, so
that the offset and segment addresses having a shift of 4 bits are added in the adder
153C to provide a physical address of 20 bits, by which a memory space of 1 M bytes
is accessed, as shown in Fig. 11. This is carried out by the physical address calculation
conduct signal 140C.
[0034] Secondly, it is assumed that the second segment register is used, and the segment
address selecting signal 143A' of "01" is supplied to the decoder 154G of the segment
address selector 154. As a result, the second bit shifter 154E is selected to provide
the segment address of 24 bits having added upper and lower 4 bits to the second bit
shifter 153B of the physical address calculation processor 153. Then, the offset and
segment addresses having a shift of 8 bits are added in the adder 153C to provide
a physical address of 24 bits, by which a memory space of 16 M bytes is accessed,
as shown in Fig. 12.
[0035] Thirdly, it is assumed that the third segment register is used, and the segment address
selecting signal 143A' of "10" is supplied to the decoder 154G of the segment address
selector 154. As a result, the third bit shifter 154F is selected to provide the segment
address of 24 bits having added lower 8 bits to the second bit shifter 153B of the
physical address calculation processor 153. Thus, the offset and segment addresses
having a shift of 12 bits are added in the adder 153C to provide a physical address
of 28 bits, by which a memory space of 256 M bytes is accessed, as shown in Fig. 13.
[0036] As explained in the first and second preferred embodiments, the number of bits for
a shift between offset and segment addresses can be varied dependent on a segment
override prefix command to access memories of various sizes by use of programs. Consequently,
a program prepared for accessing a small memory space can be used for accessing a
large memory space. At the same time, the program prepared for accessing the small
memory space can be used in combination with a program for a large memory space.
[0037] Although the invention has been described with respect to specific embodiment for
complete and clear disclosure, the appended claims are not to be thus limited but
are to be construed as embodying all modification and alternative constructions that
may occur to one skilled in the art which fairly fall within the basic teaching herein
set forth.